Previous studies (Berberich, 1985, J. Bacteriol. 163, 1109) suggested that further metabolism of some effector D-amino acids might result in the generation of a specific "signal" molecule which could elicit the ammonium nitrogen-limited stress response. Although mass spectrometric measurements revealed that the principal recipient of the amino nitrogen of each of these D-amino acids is L-serine probably via increased serine hydroxymethyltransferase activity, SHMT (Berberich and DeMoll, 1988. Abstr., ASM, 228), the conversion of D-glutamate, D-lysine or D-threonine to glycine is not as yet understood. It has been determined that D-threonine is not a substrate or an inducer or threonine dehydrogenase (TDH), threonine dehydratase, or 2-amino-3-kerobutyrate ligase, enzymes which function in the threonine utilization cycle (TUT) generating glycine and serine (Ravnikar and Somerville, 1987, J. Bacteriol. 169, 2611). That the D-amino acids may be substrates of a specific D-amino acid dehydrogenase which also produces a 2-amino-3-keto acid will be investigated since the D-amino acids are without effect when added to anaerobically cultured cells. It was observed that strains carrying mutations in glyA, the gene for SHMT, are deficient in response to the D-amino acids. When added simultaneously with the D-amino acids, each of the following: L-serine, adenine (but not hypoxanthine), glutamine, azaserine, trimethoprim; eliminates the D-amino acid response indicating that the effective consequence of increased serine synthesis via SHMT may involve the availability of glycine, formyl donors and/or purine regulation. Because about 30% of the spontaneous, second site suppressors of glnD mutations demonstrated high constitutive levels of GS and could be genetically located at glnB (Berberich, 1987, Abstr., ASM, 203), glnB-lacZ fusion strains were constructed. It was determined that in the absence of the glnB gene product, nitrogen control is eliminated. The level of GS in the fusion strains remains repressed upon addition of D-amino acids or upon growth under nitrogen-limited growth conditions. Studies on the regulation of glnB are in progress.